Although it will be evident to those skilled in the art that electrical contact assemblies as described herein have broad applicability in the electrical connector field, the electrical contact assemblies will be described in terms of a specific context, namely, cardiac pacemakers for providing precisely controlled stimulation pulses to the tissue of the heart.
IMDs are in use providing electrical pulses to stimulate body tissue via one or more electrical leads extending between the IMD and the tissue to be stimulated. The leads typically provide bidirectional electrical communication between the IMD's pulse generator and the body tissue, transmitting stimulation pulses to the tissue and sensed electrical signals generated by the tissue to the pulse generator. An example of this type of technology is a pacemaker and a pacing lead which provides electrical stimulation to the heart. The pacemaker is usually implanted in a subcutaneous cavity and the lead or leads extend either transvenously to the internal cavities or chambers of the heart or to patch electrodes affixed to an external surface of the heart.
A pacing lead generally includes at least one electrode on a distal end of the lead and an electrical connector assembly on the proximal end of the lead for connecting the at least one electrode to the pacemaker. Most of today's connector assemblies conform to industry wide standards such as the IS-1 and VS-1 standards. The connector assembly on the proximal end of the lead and the at least one electrode are connected by an electrical conductor extending within an insulated body of the lead. It is common practice for the leads to include two or more electrodes and two or more corresponding electrical contacts on the electrical connector assembly. A typical lead in use today is the bipolar lead carrying a tip electrode and a ring electrode on the distal end of the lead and an in-line electrical connector assembly on the proximal end of the lead. The connector assembly has a pin terminal contact electrically connected to the tip electrode by means of a first coil or cable conductor and a ring terminal contact electrically connected to the ring electrode by a second coil or cable conductor.
The connector assembly is received within a receptacle in a header forming part of the pacemaker. The header is typically made from an epoxy material that is assembled and bonded to the main body of the pacemaker. The main body of the pacemaker is generally a metallic, self-contained, hermetically sealed housing enclosing a battery and electronic circuitry for generating and controlling the timing of the electrical stimuli delivered by the lead.
Electrical contact elements such as toroidal shaped extension springs (usually referred to as garter springs) are mounted within the receptacle of the header to make electrical contact with the pin and ring terminal contacts on the lead connector assembly. Generally, as is the case with garter contacts, the electrical contact elements in the header are passive in that they rely on the deformation of the contact elements under load to maintain contact with the electrical terminal contacts carried by the connector assembly on the lead. Electrical signals must be reliably transferred by these contact elements bidirectionally between the body tissue to be stimulated and the circuitry of the pulse generator. Low electrical resistance contacts are especially important to reliably transfer the low level sensed signals generated by the body tissue. The reliability of the contact system must be maintained under multiple insertion/withdrawals of the lead connector assembly, and the electrical contact element within the pacemaker receptacle must be sufficiently flexible to accept a range of lead sizes so as to accommodate the dimensional tolerances of the connector assembly.
It is further important for the electrical connector assembly on the proximal end of a lead to be securely retained within the receptacle of the pacemaker to prevent inadvertent decoupling. In addition, the insertion and withdrawal forces applied to the electrical connector assembly must be maintained below certain prescribed levels. Preferably, the force required to insert the electrical connector assembly into the pacemaker receptacle should be minimized. There are ISO industry standards governing the maximum allowable insertion force of leads.
It is furthermore important that the electrical contact elements within the pacemaker receptacle not be damaged or pulled out of the receptacle by the connector assembly. Existing garter spring designs in particular have been known to roll or pop out of their retaining grooves or to otherwise become damaged as a result of repeated insertions and withdrawals of connector assembly.